Foundation units are well known. Typical foundation units include a rigid frame and a top planar portion disposed above the frame by numerous spring elements. The top planar portion is typically defined by a welded wire grid. The spring elements are typically stapled to the wooden frame below and clipped or welded to the wire grid above. Typically a pad is placed over the top planar portion and the entire assembly is surrounded with a cover.
Foundation units which use modular springs for the spring elements are also known. For example, a modular spring and a foundation unit based on that modular spring are shown in U.S. Pat. No. 3,833,948, assigned to the assignee of the present invention, the disclosure of which is fully incorporated herein by reference. However, these modular springs are typically connected to the upper wire grid by clips, welding, or other means requiring special equipment and substantial labor. Consequently, the foundation unit cannot be manufactured as economically as desired.
Thus, it is desirable to be able to attach the modular springs to the planar top portion without requiring clips or welds etc. Several techniques for attaching the modular springs to the planar top portion without the use of clips or welds are shown, for example, in U.S. Pat. Nos. 4,828,233, 4,838,528, and 5,005,809. However, these patented techniques either require a series of complex bends to be made in relatively complex modular spring elements or require numerous grid wires and/or modules to make a satisfactory foundation.